Known from laid-open German Application [Offenlegungsschrift] DE102006031188 A1 is an apparatus for determining and/or monitoring a process variable. Secured on a membrane in this disclosure is an oscillatory element, which, due to the special way in which it is embodied, is excited to execute torsional oscillations. The membrane is, in such case, deflected in such a manner that it executes a transverse movement perpendicular to the membrane surface, i.e. it oscillates in the fundamental mode. The oscillatory element is embodied in such a manner and secured with two connection regions on the membrane that the two connecting regions in the case of this movement of the membrane experience differently directed force components and the oscillatory element executes a torsional movement.
Furthermore, vibronic measuring devices are known, which have a so-called oscillatory fork as the mechanically oscillatable unit. Oscillatory forks permit, on the one hand, fill level and density measurements, while, on the other hand, also enabling measurement of viscosity. The oscillatory fork is, most often, excited by means of a driving/receiving unit in the form of one or more piezoelectric elements to execute bending oscillations. In such case, the two paddles forming the oscillatory fork execute bending movements of opposite phase. For fill level measurement, the oscillation frequency is monitored. For viscosity measurement, most often, the amplitude of the oscillations is evaluated. Due to the added-mass effect because of dragged medium, application for higher viscosity media is, however, problematic.